With recent tremendous advances in wireless communication technologies, communication system techniques have also evolved over and over, and among these techniques, Long-Term Evolution (LTE) systems being standardized in the 3rd Generation Partnership Project (3GPP) standardization organization have attracted much attention as a 4th-Generation (4G) wireless communication technique.
FIG. 1 illustrates a structure of an LTE system.
Referring to FIG. 1, a wireless access network of the LTE system includes evolved Nodes B (ENBs, Nodes B, or base stations) 105, 110, 115, and 120, a mobility management entity 125, and a serving gateway (S-GW) 130. A user equipment (UE or terminal) 135 connects to an external network through the ENBs 105 through 120 and the S-GW 130.
In FIG. 1, the ENBs 105 through 120 correspond to existing Nodes B in a Universal Mobile Telecommunication System (UMTS) system. The ENBs 105 through 120 are connected with the UE 135 through a radio channel and play more complicated roles than the existing Nodes B. In the LTE system, every user traffic as well as a real-time service such as Voice over Internet Protocol (VoIP) is provided through a shared channel, requiring a device for collecting state information of UEs, such as a buffer state, an available transmit power state, a channel state, etc., and performing scheduling based on the state information. Examples of such a device may be the ENBs 105 through 120. One ENB generally controls multiple cells. For instance, to implement a transmission speed of 100 Mbps or higher, the LTE system may use, for example, orthogonal frequency division multiplexing (OFDM) as a wireless connection scheme in a bandwidth of 20 MHz. Also, adaptive modulation & coding (AMC) is used in which a modulation scheme and a channel coding rate are determined depending on a channel state of a UE. The S-GW 130 is a device for providing a data bearer, and generates or removes the data bearer under control of the MME 125. The MME 125 is in charge of various control functions as well as a mobility management function for the UE, and is connected with the plurality of ENBs 105 through 120.
FIG. 2 illustrates a wireless protocol structure in an LTE system.
Referring to FIG. 2, the wireless protocol of the LTE system includes packet data convergence protocol (PDCP) layers 205 and 240, radio link control (RLC) layers 210 and 235, and medium access control (MAC) layers 215 and 230. The PDCP layers 205 and 240 manage operations such as compression/decompression of IP headers, and the RLC layers 210 and 235 reconfigure PDCP packet data units (PDCP PDUs) into a proper size. The MAC layers 215 and 230 are connected to several RLC-layer devices formed in one UE, and perform an operation of multiplexing RLC PDUs to a MAC PDU and de-multiplexing a MAC PDU into RLC PDUs. Physical (PHY) layers 220 and 225 channel-code and modulate upper layer data into OFDM symbols and transmit the OFDM symbols over a wireless channel; or demodulate and channel-decode OFDM symbols received over a wireless channel and transfer the decoded OFDM symbols to their upper layers. For further error correction in a physical layer, Hybrid ARQ (HARQ) is used, in which a reception end transmits 1 bit as information about whether a packet transmitted from a transmission end has ben received. This information is referred to as HARQ ACK/NACK information. Downlink (DL) HAQR ACK/NACK information for UL transmission is transmitted through a physical hybrid-ARQ indicator channel (PHICH) physical channel, and uplink (UL) HARQ ACK/NACK information for DL transmission is transmitted through a physical uplink control channel (PUCCH) or a physical uplink shared channel (PUCCH) physical channel.
In the LTE system, techniques for supporting various terminal types have been introduced, one of which supports a machine-type communication (MTC) UE. The MTC UE refers to a machine that performs communication for itself (e.g., on a time once per month) instead of being directly manipulated by a person such as an electric meter or a water meter, and also refers to a device to which connection may be attempted with low priority according to the aforementioned examples.
Among the MTC UEs, UEs used for the same purpose as the aforementioned meter do not need a high-performance data transmission capability and may have a low transmit power. The MTC UE may be installed in a poor communication environment such as an underground or a warehouse, even if having the same reception performance as a general UE. Hence, a need has emerged to distinguish a separate UE type requiring a coverage extension (CE) or an extended coverage (EC) function for overcoming a low transmit power with a low transmission speed. To this end, LTE release 12 (release refers to version information and higher version refers to newer version) newly defines the aforementioned type UE as Category #0 in addition to existing UE categories (or category is classified according DL/UL transmission speeds of the UE, and for example, a UE corresponding to Category #4 supports a DL speed of 150 Mbps and a UE corresponding to Category #5 supports a DL speed of 300 Mbps). The UE corresponding to Category #0 has a low transmission speed (e.g., 1 Mbps), and may operate in a CE mode to secure a broad coverage in spite of a low transmission power. For this end, a separate additional transmission method may be used. The separate additional transmission method may be repeated transmission or the like. Thus, the UE corresponding to Category #0 may be used as an MTC UE.
Meanwhile, if the MTC UE needs a broad coverage, the CE mode may be applied for application of the separate additional transmission method (e.g., repeated transmission, etc.) to every data transmitted and received by the MTC UE. For example, a network sends a paging message for waking up sleeping UEs to allow the UEs to receive incoming calls (or receive phone calls). This scheme is applied to every UE as well as the MTC UE. However, if the network has no information about the MTC UE, the BS, when sending the paging message, does not know whether a UE to receive the paging message is a normal UE or an MTC UE, inevitably having to send every message by using the additional transmission (e.g., repeated transmission, etc.,). Such unnecessary repeated transmission results in waste of radio resources, and a resource that needs to be used for data transmission is used for control message transmission, degrading a transmission speed. Therefore, this problem has to be solved.
Another problem is that the MTC UE has difficulty in using an existing method when performing cell selection and re-selection. The performance of a receiver of the MTC UE may be lower than that of a normal UE, and even if having the same receiver performance as the normal UE, the MTC UE is highly likely to be installed in a poorer environment than with the normal UE. As a result, when attempting cell selection using an existing method, the MTC UE may not find a proper serving cell. In another way, if the MTC UE is capable of selecting a serving cell by using an existing method, it may be desirable to avoid searching for a serving cell in a CE mode. This is because the MTC UE is provided with a lower-quality and lower-efficiency service than in an existing UE in the CE mode. In cell re-selection, the UE re-selects a serving cell based on frequency priority information provided from the BS. In this case, when re-selecting each frequency, the MTC UE needs to consider connection in the CE mode.